Field of the Invention
This invention relates generally to a method and apparatus for processing an audio signal and, more particularly, to processing an audio signal so that the resultant sounds appear to the listener to emanate from a location other than the actual location of the loudspeakers.
Human listeners are readily able to estimate the direction and range of a sound source. When multiple sound sources are distributed in space around the listener, the position of each may be perceived independently and simultaneously. Despite substantial and continuing research over many years, no satisfactory theory has yet been developed to account for all of the perceptual abilities of the average listener.
A process that measures the pressure or velocity of a sound wave at a single point, and reproduces that sound effectively at a single point, will preserve the intelligibility of speech and much of the identity of music. Nevertheless, such a system removes all of the information needed to locate the sound in space. Thus, an orchestra, reproduced by such a system, is perceived as if all instruments were playing at the single point of reproduction.
Efforts were therefore directed to preserving the directional cues contained inherently in the sounds during transmission or recording and reproduction. In U.S. Pat. No. 2,093,540 issued to Alan D. Blumlein in September 1937 substantial detail for such a two-channel system is given. The artificial emphasis of the difference between the stereo channels as a means of broadening the stereo image, which is the basis of many present stereo sound enhancement techniques, is described in detail.
Some known stereo enhancement systems rely on cross-coupling the stereo channels in one way or another, to emphasis the existing cues to spatial location contained in a stereo recording. Cross-coupling and its counterpart crosstalk cancellation both rely on the geometry of the loudspeakers and listening area and so must be individually adjusted for each case.
It is clear that attempted refinements of the stereo system have not produced great improvement in the systems now in widespread use for entertainment. Real listeners like to sit at ease, move or turn their heads, and place their loudspeakers to suit the convenience of room layout and to fit in with other furniture.